Temporal variation in intensity of upwelling off southwest Africa

Abstract

Abstract Sediments recovered at DSDP Sites 362 and 532 on Walvis Ridge Abutment Plateau and 530 in the southeastern Angola Basin record long-term waxing and waning of upwelling. The amounts of opaline silica and organic carbon in the sediments increase from latest Miocene to latest Pliocene then decline to present. During the late Pliocene, opaline silica accumulated ten times faster than during the late Pleistocene. If the area of accumulation also expanded to include the Abutment Plateau, this region would have been the global silica sink, consuming the equivalent of the entire silica output of the present ocean. The sediments contain light-dark cycles; the dark cycles contain more terrigenous material and on this basis have been interpreted as representing glacials. Before the Pliocene the maximum biological productivity in this region occurred during the glacials, but since then it has occurred during the interglacials, an effect of sea-level change on productivity opposite to that in most parts of the world. The most important factors causing the changes in sedimentation are judged to be threefold. (1) Closing of the Central American Isthmus resulted in salinization of the North Atlantic and caused increased production of NADW, AAIW and AABW. The increased rate of production of these water masses resulted in differentiation of nutrients among them and may have resulted in a shallower pycnocline facilitating upwelling of nutrient-rich water. The production of relatively nutrient-rich AAIW was a prerequisite for the long term increase in productive upwelling recorded by the late Miocene to latest Pliocene sediments. (2) During the Late Miocene northward migration of the subtropical high and ITCZ in response to growth of the Antarctic ice cap may have initiated productive upwelling over the Abutment Plateau. Subsequent southward migration of the ITCZ as the Earth changed from unipolar to bipolar glaciation would have enhanced the Angola Coastal Current and upwelling from the Angola Thermal Dome. A stronger Angola Coastal Current may have carried productive waters from the Angola Dome and focussed advection of productive coastal waters upwelled along the Namibian shelf over the Walvis Ridge Abutment Plateau. The decline in strength of upwelling since 1.7 Ma recorded by the sediments on the Abutment Plateau may reflect southward shift of the upwelling center to its present location near Lüderitz in response to the growth of northern hemisphere ice caps. (3) The desiccation of the Mediterranean drew the ITCZ to its maximum northern position and the reflooding corresponded to the time of change from maximum upwelling productivity coinciding with glacials to its coinciding with interglacials. During glacials, when NADW production was reduced, AAIW was replaced by nutrient-poor NAIW, so that the upwelled water did not become productive. Another possible cause of the general decline in productivity since 1.7 Ma is the that increased Mediterranean saline outflow has resulted in progressive expansion of NAIW into the South Atlantic during glacials.